The invention relates to a simple process for producing amino acid moiety-containing organosilicon compounds.
Amino group-containing organosilicon compounds play an important role in industry. The presence of the polar amino group in a polysiloxane significantly increases the interaction with polar surfaces and thus the adhesiveness of the polysiloxane for example. This gives rise to a multitude of possible industrial applications, for example in the field of textiles finishing or in cosmetics. In this connection amino acid-functionalized organosilicon compounds are very interesting because the additional presence of the carboxylic acid moiety can achieve a much higher still polarity. A further positive aspect is that the production thereof can utilize the broad and also cost-effective feedstock base of the industrially available amino acids. The very predominant portion of the amino acids is generated from renewable feedstocks and the product class of organosilicon compounds containing an amino acid moiety therefore also offers advantages from the aspect of sustainability.
Various processes for preparing amino acid-functionalized polysiloxanes are already known. It is amino acid-functional organosilicon compounds where the basic character of the amino moiety is retained even after linking with the polysiloxane that are primarily of interest. This is the case particularly for the linking of the amino acid with the polysiloxane by addition onto a reactive epoxide moiety on the polysiloxane.
According to this principle EP 2826806, EP 2231752, DE 10036532 and JP 52-114699 describe various production variants for amino acid-functional polysiloxanes.
In the prior art the reactions are always performed with 1 mol of amino acid per mole of epoxide moiety. Following this technical teaching however affords products which have poor solubility or have insoluble constituents.
However, for industrial applicability of amino acid-functional siloxanes in various fields of application solubility plays an important role. Poorly soluble or even insoluble products cannot be applied uniformly or in a desired low concentration for example. This often results in increased consumption and is therefore uneconomic. In addition, purification of the products is impeded enormously. Inhomogeneities can result in attenuation or loss of the industrially required properties or else can have undesired side effects.
Insufficient solubility is a problem for the industrially interesting higher molecular weight systems in particular, since solubility generally decreases with increasing molecular weight.
The solubility problem mentioned is circumvented in EP 2826806 when the amino acid moiety is introduced as a salt of an organic cation, for example a quaternary ammonium or phosphonium cation having a long-chain alkyl radical, which increases the solubility of the system. A great disadvantage here is that in the process the amino acids must initially be converted in a two-stage process into the quaternary ammonium salts or the phosphonium salts before the actual reaction with the polysiloxane is effected. In addition, the quaternary ammonium/phosphonium compounds are extremely costly components. The biocidal activity and poor biodegradability of the quaternary ammonium salts also makes the use of this compound class controversial.
In the process described in EP 2231753 the solubility problem is circumvented by performing the reaction a priori in heterogeneous phase in aqueous emulsion in the presence of an emulsifier. This process preferably employs polyether moiety-containing polysiloxanes which greatly increase the propensity for formation of stable emulsions.
A disadvantage of this process is that the formed amino acid group-bearing siloxane is always generated in admixture with an emulsifier which cannot be removed from the product. The obtained products cannot be isolated in pure form according to this teaching. This restricts the field of application.